Glomerular ultrafiltration and tubular reabsorption of bovine serum albumin and derivatives with increased negative charge in the normal female Wistar rat

Receptor-Mediated Melanoma Targeting with Radiolabeled α-Melanocyte-Stimulating Hormone: Relevance of the Net Charge of the Ligand

A majority of melanotic and amelanotic melanomas overexpress melanocortin type 1 receptors (MC1Rs) for α-melanocyte-stimulating hormone. Radiolabeled linear or cyclic analogs of α-MSH have a great potential as diagnostic or therapeutic tools for the management of malignant melanoma. Compounds such as [¹¹¹In]DOTA-NAP-amide exhibit high affinity for the MC1R in vitro, good tumor uptake in vivo, but they may suffer from relatively high kidney uptake and retention in vivo. We have shown previously that the introduction of negative charges into radiolabeled DOTA-NAP-amide peptide analogs may enhance their excretion and reduce kidney retention. To address the question of where to place negative charges within the ligand, we have extended these studies by designing two novel peptides, Ac-Nle-Asp-His-d-Phe-Arg-Trp-Gly-Lys(DOTA)-d-Asp-d-Asp-OH (DOTA-NAP-d-Asp-d-Asp) with three negative charges at the C-terminal end (overall net charge of the molecule -2) and DOTA-Gly-Tyr(P)-Nle-Asp-His-d-Phe-Arg-Trp-NH₂ (DOTA-Phospho-MSH_2-9) with two negative charges in the N-terminal region (net charge -1). The former peptide showed markedly reduced receptor affinity and biological activity by >10-fold compared to DOTA-NAP-amide as reference compound, and the latter peptide displayed similar bioactivity and receptor affinity as the reference compound. The uptake by melanoma tumor tissue of [¹¹¹In]DOTA-Phospho-MSH_2-9 was 7.33 ± 0.47 %ID/g 4 h after injection, i.e., almost equally high as with [¹¹¹In]DOTA-NAP-amide. The kidney retention was 2.68 ± 0.18 %ID/g 4 h after injection and hence 44% lower than that of [¹¹¹In]DOTA-NAP-amide. Over an observation period from 4 to 48 h, the tumor-to-kidney ratio of [¹¹¹In]DOTA-Phospho-MSH_2-9 was 35% more favorable than that of the reference compound. In a comparison of DOTA-NAP-d-Asp-d-Asp, DOTA-Phospho-MSH_2-9 and DOTA-NAP-amide with five previously published analogs of DOTA-NAP-amide that altogether cover a range of peptides with an overall net charge between +2 and -2, we now demonstrate that a net charge of -1, with the extra negative charges preferably placed in the N-terminal region, has led to the lowest kidney uptake and retention. Charges of +2 or -2 markedly increased kidney uptake and retention. In conclusion, the novel DOTA-Phospho-MSH_2-9 may represent a new lead compound for negatively charged linear MC1R ligands that can be further developed into a clinically relevant melanoma targeting radiopeptide.

Neuroendocrine tumor targeting: study of novel gallium-labeled somatostatin radiopeptides in a rat pancreatic tumor model

Somatostatin analogs labeled with radionuclides are of considerable interest in the diagnosis and therapy of SSTR-expressing tumors, such as gastroenteropancreatic, small cell lung, breast and frequently nervous system tumors. In view of the favorable physical characteristics of the Ga isotopes (67)Ga and (68)Ga, enabling conventional tumor scintigraphy, PET and possibly internal radiotherapy, we focused on the development of a Ga-labeled somatostatin analog suitable for targeting SSTR-expressing tumors. For this purpose, 3 somatostatin analogs, OC, TOC and TATE were conjugated to the metal chelator DOTA and labeled with the radiometals (111)In, (90)Y and (67)Ga. They were then evaluated for their performance in the AR4-2J pancreatic tumor model by testing SSTR2-binding affinity, internalization/externalization in isolated cells and biodistribution in tumor-bearing nude mice. Surprisingly, we found that, compared to (111)In or (90)Y, labeling with (67)Ga considerably improved the biologic performance of the tested somatostatin analogs with respect to SSTR2 affinity and tissue distribution. (67)Ga-labeled DOTA-somatostatin analogs were rapidly excreted from nontarget tissues, leading to excellent tumor-to-nontarget tissue uptake ratios. Of interest for radiotherapeutic application, [(67)Ga]DOTATOC was strongly internalized by AR4-2J cells. Furthermore, our results suggest a link between the radioligand charge and its kidney retention. The excellent tumor selectivity of Ga-DOTA somatostatin analogs together with the different applications of Ga in nuclear oncology suggests that Ga-DOTA somatostatin analogs will become an important tool in the management of SSTR-positive tumors.

An isolated perfused rat kidney preparation designed for assessment of glomerular permeability characteristics

The aim of the present investigation was to modify the widely used isolated perfused rat kidney preparation to make it more suitable for studies of glomerular permeability to macromolecules. Both kidneys were perfused in situ using separate pumps in two of each other independent systems with Tyrode-solution containing human serum albumin (18.2 g l-1). Sodium nitroprusside was administered to induce dilatation and to maintain constant vascular resistance (PRU100) during the experiments. The addition of sodium nitroprusside decreased vascular resistance from 0.17 +/- 0.05 to 0.09 +/- 0.02 mmHg min-1 100 g-1 ml-1 and increased urine flow and glomerular filtration rate. The temperature of the perfusate was reduced from 37 degrees C to 8 degrees C to inhibit tubular reabsorption of protein and fluid, resulting in a urine to plasma concentration ratio of [51Cr]EDTA of 1.26 +/- 0.07. Furosemide reduced the urine to plasma concentration ratio for [51Cr]EDTA further to 1.15 +/- 0.02 and increased glomerular filtration rate. Moreover, by performing the studies at low temperatures (8 degrees C) in the presence of sodium nitroprusside and furosemide it was possible to achieve low and stable albumin fractional clearance values close to those prevailing in vivo. Thus, the described technique, allowing simultaneous perfusions of both kidneys with different solutions, pressures and flows, seem to be well suited for studies of macromolecular transport across glomerular capillaries.

The renal uptake of proteins: a nonselective process in conscious rats

The selectivity of the renal reabsorption of proteins has been investigated by competition experiments in conscious rats. The animals were intravenously injected with increasing doses of proteins over a wide range of net charge and size, including lysozyme, cytochrome C, metallothionein, beta 2-microglobulin, retinol-binding protein, albumin and IgG. The urinary excretion of exogenous proteins injected concomitantly (human beta 2-microglobulin, retinol-binding protein, albumin and/or egg white lysozyme depending on the experiment) and of rat beta 2-microglobulin, albumin and IgG was determined with specific immunoassays. The results show that low molecular weight cationic proteins and low or high molecular weight anionic proteins can increase each other's urinary excretion. Several observations strongly suggest that these effects result from a competitive inhibition of renal uptake. The phenomenon is dose-related in most cases and, as evidenced by cytochrome C injection, transient, reproducible and saturable. In addition, the injected proteins induce a tubular type proteinuria irrespective of their net charge and size. In the case of cationic proteins, this finding excludes the possibility of an enhanced glomerular permeability due to a partial neutralization of the glomerular polyanion which, as demonstrated with protamine sulfate, entails a glomerular type proteinuria. These quantitative data on the mutual inhibition of renal uptake of a wide spectrum of specific proteins lead us to challenge the concept of charge- and size-selective tubular reabsorption of proteins, and to postulate that proteins filtered through the glomeruli are taken up by common tubular endocytotic sites irrespectively of their physicochemical features. As demonstrated by the ability of beta 2-microglobulin and IgG to inhibit the uptake of lysozyme, the affinity of a protein for reabsorption sites is not simply related to its size and net positive charge. Evidence is also presented that proteins, when administered intravenously at high doses, induce a lysosomal enzymuria most likely reflecting a stimulated exocytosis.

Glomerular permeability in the isolated perfused rat kidney

Increased glomerular permeability to protein occurs during isolated rat kidney perfusion in the absence of ultrastructural changes in the glomerular capillary wall. The increase is greater with age, varies with strain and is reduced by amino acid supplementation of the perfusate. Glomerular permeability to protein is thus perceptibly influenced by non-pathogenic stimuli. Such phenomena, in turn, may influence glomerular function in disease or after experimental damage.

Observations on the effect of immersion in Bath spa water

Immersion in water in spas has been practised for centuries and has many proponents. Despite fierce debate about its efficacy there has been little scientific evaluation of the effect of immersion in mineral waters. Eight normal subjects were immersed in Bath spa water for two hours and the renal, haematological, and cardiovascular responses were compared with those in the control periods before and after immersion. Significant, twofold diuresis and natriuresis, 5% haemodilution, and a 50% increase in cardiac index were observed in subjects immersed, sitting, in Bath spa water at 35 degrees C. These changes may constitute part of the scientific rationale for spa treatment in many states of disease.